Microbial desulfurization offers a potential new approach to removing sulfur compounds from fuel oils with minimal environmental impacts (Gray et al., (1996) Nature Biotechnology 14, 1705-1709). Progress has been made in isolating the genes for the individual enzymes in the desulfurization pathway, and studying their function in recombinant E. coli. We are using sulfur K-edge spectroscopy to follow the desulfurization of fuel oils by these organisms in an attempt to understand their substrate specificity in complex mixtures.